Thin-film oxide semiconductors are being studied for use as a channel layer of a transistor. In particular, ZnO oxide-semiconductors are being studied actively for use as a channel of a thin-film transistor (TFT).
However, the thin ZnO film as a semiconductor film, when formed at room temperature, will become polycrystalline and will have several nanometers to several tens of nanometers of roughness at a semiconductor-insulator interface, a semiconductor-semiconductor interface, or a semiconductor-metal interface: the interface is highly important in the semiconductor device.
Further, in the polycrystalline matter, crystal grain boundaries will inevitably be formed. The grain boundary will cause defects in conduction or will cause deterioration of the property owing to adsorption of gas molecules from the atmosphere to cause instability of the characteristics, disadvantageously (Journal of Applied Physics, Vol. 94, p. 7768).
To overcome the aforementioned disadvantages resulting from the rough interface or the grain boundaries, U.S. Patent Application Publication No. 2007/0194379 discloses a high-performance TFT which employs an amorphous oxide semiconductor as a thin-film of an oxide semiconductor.
The semiconductor formed in an amorphous state gives an excellent interface having roughness of less than a nanometer without the roughness of the interface of ZnO, enabling a higher performance of the semiconductor device. Thereby, a semiconductor film can be formed without grain boundaries, which prevents deterioration and instability of the properties caused by the grain boundaries.
Therefore, an amorphous oxide semiconductor film and a TFT containing this semiconductor film were produced by pulse laser vapor deposition (PLD) under the same conditions as in the above U.S. Patent Application Publication No. 2007/0194379. The obtained semiconductor film and the TFT had properties equivalent to the properties shown in that U.S. patent document, or reported in Nature, Vol. 432, p. 488.
Further, an amorphous oxide semiconductor film and TFT were produced by sputtering by use of a target of the composition InGaO3(ZnO), namely an oxide of In1Ga1Zn1. Thereby, an amorphous oxide semiconductor film and a TFT film were obtained which had as excellent properties as the one produced by the PLD process (Applied Physics Letters, Vol. 89, pages 112123-1 to 112123-3).
After further investigation, the inventors of the present invention have found the conditions for producing the amorphous oxide semiconductor having further improved properties of the semiconductor and TFT.
The present invention provides an amorphous oxide semiconductor containing at least one element selected from In, Ga, and Zn; and a semiconductor device and a thin-film transistor employing the amorphous oxide semiconductor.